Electric furnaces have been used for many years to melt thermally fusible material such as ores, slags, glasses, oxides, rocks and oxidic waste materials, and a number of different furnace designs have been developed in an attempt to optimize the melting of the batch. In the case of ores and metallic-containing materials, graphite or carbon electrodes have been used in various combinations and configurations including single phase and multiphase operations with the electrodes either in an in-line configuration or in variations of a central delta-type arrangement. In the case of glass or oxide melting, deeply immersed electrodes of molybdenum, tungsten or tin oxide have been used, again in various geometrical and electrical phasing configurations.
When graphite or carbon electrodes are employed they are usually carried well above the liquid melt line, with the heat from the electrode arcs being absorbed by the surrounding batch or charge material. Furnace in which the depth of the batch material surrounding the electrode tips is over about six inches are known as "submerged arc" furnaces to characterize the fact that the arc column from the electrode tip to the melt surface is submerged by the batch material. One characteristic of submerged arc furnaces is that the electrode tips are positioned at a distance greater than 1/2 inch above the melt surface, usually on the order of 4 to 10 inches. This ensures that in such furnaces the heat is transferred directly from the arc column to the charge material rather than indirectly first to the melt surface and then to the charge material. While this configuration results in an efficient usage of heat, it is necessary that the charge material used in submerged arc furnaces be carefully prepared in size and consistency to allow reaction gases to safely escape as the charge column gradually melts and descends in the furnace.
In contrast to submerged arc furnaces, it is known to be able to operate furnaces with graphite or carbon electrodes immersed within the molten slag layer. However, immersion of uncooled carbonaceous electrodes more than about 2 inches in the melt is generally not desirable because of rapid reaction of the carbonaceous electrode material with the melt, giving rise to excessive electrode consumption. Cooling of carbonaceous electrodes as suggested in U.S. Pat. No. 2,591,709 to Lubatti is not a satisfactory method of obtaining deeper immersions because of excessive electrode skulling by the molten material and excessive heat losses to the electrode cooling liquid.
To utilize the advantage of close electrode coupling with the melt, but without undue electrode wear, it was specified in U.S. Pat. Nos. 2,805,929 and 2,805,930 to Udy that the electrode tips must be positioned from 1/2 inch above to no more than 2 inches below the melt surface. A formula was further developed in U.S. Pat. No. 3,522,356 to Olds et al. for the exact placement of the electrode tips according to the Udy configuration. The Olds et al. patent further noted than the electrical discharge from electrodes positioned in this manner was a corona-type discharge rather than an arc discharge.
It has been known, as pointed out in U.S. Pat. No. 2,744,944 to Striplin, Jr. et al. that the operation of submerged arc furnaces could be improved by slowly rotating the furnace shell while keeping the roof and electrode columns stationary. A number of submerged arc furnaces began to include shell rotation principally to allow the use of an increased amount of fine batch material in the furnace charge. It was felt that the very slow rotation kept the fine material from premature sintering, thus allowing the charge to be melted in a controlled manner rather than by causing the disastrous explosions frequently observed in nonrotating ferroalloy furnaces. The development of the rotating shell for such furnaces was directed exclusively toward a relative interaction between the electrodes and the surrounding batch material rather than between the electrodes and the molten bath. As a result, shell rotation times were very long, typically one to two days to complete a single revolution. This corresponds to angular speeds of from 0.1 to 0.2 degrees per minute.
In the case of glass and oxide melting, completely immersed metal and tin oxide electrodes have generally been employed rather than carbonaceous electrodes. For such melting applications, the electrodes have had many different shapes, including both rectangular and round, and have been placed in many different configurations with respect to each other. These electrodes have been made to be laterally or vertically adjustable as a means for altering melting conditions, and they have been designed to be inserted through the top, through the side walls or through the bottom of the furnaces. Examples of one or more of these features can be found in U.S. Pat. Nos. 2,089,690 to Cornelius, 2,686,821 to McMullen, 3,539,691 to Lucek, and 3,983,309 to Faulkner et al.
In addition, U.S. Pat. No. 4,351,054 to Olds discloses an arrangement which provides for optimal spacing of such immersed electrodes both laterally and vertically with respect to each other. The electrodes are mounted on support arms which extend over a furnace vessel with an open top, and the arms themselves are mounted for horizontal and vertical adjustment to enable the electrodes to be precisely positioned. The ability to locate the electrodes in their ideal location, taking into account such variables as the size of the furnace vessel, the magnitude of electrical power employed and the desired working temperature of the furnace results in improved melting rates and increased melter life.
Other means for developing improved melter performance have involved the use of the electrodes as mechanical agitators and stirring apparatus, examples of which can be found in U.S. Pat. Nos. 4,055,408 to Novak et al., 3,819,350 to Pellet et al., and 3,539,691 to Lucek. Such mechanical means for agitating and mixing melts have the obvious disadvantage of requiring electrical rotors for transferring electrical energy to the rotating electrode column. Such rotors are difficult to maintain, especially around hot, aggressive melter environments. Further, the viscous melts are difficult to move mechanically and require considerable amounts of energy to effect meaningful homogenization over the entire melter area.
Many other concepts for rotating either melter shells or ancillary equipment or both have been proposed from time to time, such as in U.S. Pat. No. 4,676,819 to Radecki et al. All of these proposals, however, fail to adequately homogenize the melt in the melter itself. What is needed is a simple economic means for improving the mixing and for more fully homogenizing the melt without the difficulties imposed by the suggestions of the prior art.